This is a test to see how well you know your boat. Your boat has a 50 amp 220 volt service. Do you have any 220 volt appliances, air conditioners, water heater, or stove?

If not, one of these will allow you to use two 30 amp plugs. I have one; it works perfectly for my boat; and it doesn't matter if the pluge are on the same phase or not and whether the combined voltage is 208 or 220.

Ted

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Cottontop,
As Ted said above, a lot depends on your boat - what you have aboard and how it is wired.
Our boat has a Charles Iso-Boost and is wired such that it needs 50amp or nothing.
For us, the device Ted can use, would not work for us.
We used a Smart-Y many times on the loop. In addition to the Smart-Y, we found a second 50amp cord came in handy, along with three 30amp cords (allowing us to get to a different phase 30amp outlet).
Henry

As I understand it, a reverse Y will not work if the power source is from the new ELCI / RCD marina power outlet.

Probably so, but then if the marina is sporting ELCI breakers, it probably has upgraded the power wiring, so I'd expect that more pedestals would be fitted with 50A receptacles, probably negating the need for the smart wye anyway.

Am I likely to need a Smart Y to get from two 30 amp 125v to the single 50 amp 125/250v? Or do most (all) marinas have the 50 amp?

The Smart Y looks to be about $400.

Thanks.

In Canada, smaller marinas usually don't have 50A service and along the heritage canals electrical service is uncommon but you sometimes can get a single 30 A. I always carry a smart Y, along with an extra 50' 30A cord (and , as others have mentioned, a spare 50A cord also). Sometimes you can't get both phases on the same 30A pedestal, or only 1 30A outlet left on the nearest pedestal hence the extension.
Enjoy your loop